How to prevent mold in knee wall cabinet dressers

I have a 1950 cape that has built in knee wall dressers in each room. The cabinets are made of plywood which are poplar wood and the joints are very tight. The cabinets behind the knee wall are wrapped with 1/4 inch foil insulation to keep the cold air out. Both ends of the house behinds the knee walls are also 8x8 louver vents so there is adequate airflow behind the knee wall. The problem I just found was mold growing on the inside of the cabinets, mostly on the bottom. I assume because the wood was cold and the air from the room was warm, moisture was created which created the mold.

I need to know how to correct this problem. I have used a variety of chemicals to clean the mold but need to know how to correct the problem. I heard one option was to seal the inside of the cabinets with polyurethane. Any suggestions?